Head suspension assemblies (HSA) in rotatable data storage devices are spring structures that perform the difficult task of accurately holding and positioning a floating head assembly nanometers away from the rapidly spinning irregular surface of a rotatable data storage device. The HSA can be part of a magnetic hard disk drive, the most common today, or other type of drives such as an optical disk drive.
A HSA comprises different elements, the most common being a suspension assembly and a head assembly. A suspension assembly, the spring element, usually includes a load beam and a gimbal, each composed of a carefully balanced combination of rigid regions and flex spring regions. A typical head assembly usually includes a "head", comprising of a highly sensitive read/write transducer, that is attached to an air bearing slider.
In a magnetic disk drive, a write transducer transforms electrical pulses to small magnetic fields which it then "writes" on a magnetic disk. A read transducer decodes these magnetic fields back into electrical pulses. The order of the magnetic fields and their subsequent orientation, aligned along the circumference of the disk in north-south configuration, defines a bit code that the transducer detects as the head floats on a cushion of air over the magnetic disk. The head assembly includes electrical terminals to send and receive these electrical pulses.
A HSA generally attaches at its proximal end to a rigid arm manipulated by a linear or rotary motion actuator designed to position the head at any radial location above the disc. The spinning disk coupled with the actuator movement allows the head to gain access to multiple tracks across the disk surface, each track capable of containing large amounts of densely stored data. Positioned at the distal end of a suspension assembly, a gimbal holds the head assembly level and at a constant distance over the contours of the disk.
This gimbal is the most critical of the spring regions in a HSA. The closer the head assembly can fly to the surface of a magnetic disk, the more densely can information be stored (the strength of a magnetic field is proportional to the square of the distance, thus the closer the head flies the smaller the magnetic "spot" of information). Today's disk drives strive to reach flying clearances close to 100 nanometers=0.1 micrometers (a human hair is about 100 micrometers thick). Greater data densities allow for greater storage and smaller size. But the head assembly must not touch the disk ("crash"), as the impact with the rapidly spinning disk (rotating at about 3600 rpm or faster) could destroy both the head and the surface of the disk, along with the data stored on it.
In order to achieve this delicate and precise positioning, a suspension assembly, and specially the gimbal flexure, must carefully balance the load applied to the head assembly against the upward lift of the air stream on the slider. Since at this microscopic level the seemingly smooth surface of the disk is full of peaks and valleys, the gimbal spring must be very responsive in order to maintain a level flying height. To avoid delays and errors, it must also resist torsion and momentum forces, and maintain the head parallel to the surface even after rapid repositioning movements. The best suspension assemblies are low in mass, to reduce momentum on the floating head, and very flexible along the Z-axis, to quickly adjust to surface undulations. They also are balanced carefully to reduce static roll and pitch to acceptable levels and to avoid applying an initial twist to the head.
A conventional gimbal flexure, sometimes referred to as a Watrous gimballing flexure design, is formed from a single sheet of material and includes a pair of outer flexible arms about a central aperture, with a cross piece extending across and connecting the arms at a distal end of the flexure. A flexure tongue is joined to the cross piece and extends from the cross piece into the aperture. A free end of the tongue is centrally located between the flexible arms. The slider is mounted to the free end of the flexure tongue. The slider must be mounted to the flexure tongue so that the head assembly is in a predetermined (e.g., planar and parallel) relationship to the disk surface to assure accuracy and overall planarity.
As the head writes and reads to and from the disk, it receives and sends electrical pulses of encoded information. Complex head assemblies may require four or more different input and output terminals. The electrical signals are relayed to appropriate amplifying and processing circuitry. Signal transmission requires conductors between the dynamic "flying" slider and the static circuitry of the data channels. However, while conductors can be routed fairly easily along the rigid actuator arm, providing the final interconnections through the suspension assembly, and specially those over the gimbal to the head, is often extremely troublesome with current interconnect schemes.
Specially designed HSA interconnect assemblies are required in order to relay electrical signals accurately while not disturbing the precise balance of the HSA components. The term interconnect assembly refers to the entire interconnect system in a HSA, including conductors, insulators, and other features. In order to assure data precision, interconnect assemblies must transmit the electrical signals free from interference or signal loss due to high inductance, high capacitance, or high resistance. Optimal interconnect assemblies must be attached securely in order to reduce movement and vibration which cause varying electrical characteristics and affect mechanical performance. They must have low resistance and be well insulated from electrical ground.
Also, as technology advances, an interconnect system also must be capable of handling a plurality of signals. A basic interconnect assembly for a transducer having a single read/write inductive element calls for two conductors. More complex transducer designs may incorporate a separate magneto-resistive read element and an inductive write element, thus requiring a minimum of 3 conductors if the elements are tied together or a minimum of 4 conductors if the elements are completely separate. More advanced systems require even more conductors.
The problem is that in a HSA, interconnect assemblies must be planned around competing and limiting design considerations. The interaction of all the elements of an HSA forms a carefully balanced system. An ideal interconnect assembly must satisfy strict mechanical and manufacturing requirements.
First, an interconnect assembly must not impose unpredictable loads and biases which might alter the exact positioning of the head assembly, nor must it detract from the ability of the spring regions to adjust to variations in the surface of the disk, vibrations, and movement. Alterations to the flying height of the head can significantly affect data density and accuracy and even destroy the system in a crash. Neither of these two results would be well received by computer users.
Rigidity increases in relation to the third power of cross-sectional thickness. To respond to air stream changes and to hold a floating head, suspension assemblies are very thin and light, specially around the sensitive spring areas. A thick conductor placed atop of a thin suspension will dramatically increase a spring region's stiffness. Moreover, overshoot errors caused by inertial momentum are also affected by thick, high-in-mass conductors. Therefore, the ideal interconnect assembly must be low in mass and be very thin. As an additional requirement, interconnects placed over spring regions must not plastically deform when the region flexes, since that would hinder the return of the spring to its normal position and apply a load on the suspension assembly.
Since the gimbal region must adjust to minute variations in the surface of the disk, vibrations, and movement, this region is usually the thinnest and most delicate, and therefore the most affected. Because of the rigidity to thickness relation, even thin conductors placed atop the gimbal will greatly decrease the flexibility of a gimbal region.
A second set of design considerations comes from manufacturing constraints. Like any commercial product, interconnect assemblies must be efficient to manufacture. Additionally, they must mate well and easily with the suspension assembly, be resistant to damage, and have precise manufacturing tolerances. Complex shaping and mounting processes are costly and decrease the reliability of the whole HSA manufacturing process. Fragile electrical conductors or interconnects that have to be added to the suspension assembly early in the manufacturing process are more susceptible to damage by production steps. They drastically diminish the manufacturing useful output yield.
Exacting tolerances are necessary to avoid defects and unpredictable loads and biases, specially when dealing with such minute measurements and clearances. During the process of manufacturing and assembling the HSA and of attaching an interconnect system, any lack of precision in forming or assembling the individual elements contributes to a lack of planarity in the surfaces of the elements. A buildup of such deviations from tolerance limits in the individual elements can cause deviation from desired planar parallelism in the final head suspension assembly. The parameters of static roll and static pitch torque in the final HSA result from these inherent manufacturing and assembly tolerance buildups. As the industry transitions to smaller slider/transducer sizes to increase data storage density, limitations of the current interconnecting devices increase the potential for read/write errors and impose ceilings on data storage density.